This invention relates to filtration and, more particularly, to the use of a direct current to improve the filtration operation.
Clays such as kaolin, bentonite and the like are widely used as adsorbents, pigments, catalysts, paper fillers and the like. These clays generally occur in geological deposits as mixtures with inert foreign materials, and it is necessary to separate the clay material before the clay is utilized in commercial application.
In effecting the separation, the crude clay is dispersed in water with the aid of dispersing chemicals such as sodium silicate, sodium hexametaphosphate, sodium tripolyphosphate and tetrasodium pyrophosphate, and the clay slurry is blunged, degritted, classified and leached to effect removal of the undesired foreign materials. Following these steps, the clay is then filtered to remove the dispersing and leaching chemicals and recover the solids portion of the suspension. A 50% to 60% solids cake is then redispersed and spray dried or otherwise dried in the flocculated state.
In the filtration of clay suspensions, the conventional practice is to employ a rotary drum vacuum filter. An adequate description of such filters may be obtained by reference to any standard text as, for example, the Encyclopedia of Chemical Process Equipment; Ed. Wm. J. Mead; Reinhold Publishing Co., N.Y., N.Y.; 1964, pp. 417-38; Chemical Engineers' Handbook; J. H. Perry, Ed.; McGraw-Hill Book Co., N.Y., N.Y.; 1963, pp. 19 - 73 to 19 - 86; W. L. McCabe and J. C. Smith, Unit Operations of Chemical Engineering; 2d Ed.; McGraw-Hill Book Co., N.Y., N.Y.; 1967; pp. 880-904; A. S. Foust et al., Principles of Unit Operations; John Wiley & Sons, Inc., N.Y., N.Y.; 1960; pp. 490-500.
Rotary drum vacuum filters generally comprise a filter drum rotatably mounted in a casing or housing. A filter medium is mounted on the exterior surface of the drum and a vacuum is maintained within the drum interior. The filter drum rotates in a suspension of solid material in a liquid supplied to the lower portion of the casing. As the filter drum is rotated, the vacuum within the drum causes the liquid to pass through the filter medium and into the interior of the drum leaving a solid residium or filter cake on the filter surface. The filter cake is then removed as the drum rotates out of the suspension. The removed filter cake is then further processed as desired. The liquor or filtrate is removed from the interior of the drum and may also be processed as desired. In using such filtration equipment, the filtration produced thereby is not a perfect solids-liquid separation. The filtration actually separates the clay suspension into two parts; (1) a liquid stream or filtrate which passes through the filter medium and contains most of the suspension liquor and a low concentration of solids and (2) a filter cake which remains on the filter medium and contains a major portion of solids and a minor portion of entrained suspension liquor.
Using nylon, polyester or polypropylene filter cloths having a porosity between 0.5 to 4 cubic feet per minute in conjunction with rotary drum vacuum filters, it is possible to obtain cake soids in the order of 60% with filtrate solids on the order of 2% or less at a rate of approximately 15 pounds of dry clay per hour per square foot of filter area. The 60% solids cake is then redispersed and spray dried or otherwise dried in the flocculated state.
As it is for economic reasons desirable in the clay industry to ship slurries containing 70% solids, it is conventional in the clay art to add approximately 35% to 50% by weight spray dried clay to a 50% to 60% solids redispersed filter cake to yield a 70% solids slurry. The addition of spray dried clay to dispersed clay filter cake to prepare a 70% solids slurry for shipment adds significantly to the cost of the shipped slurry due to the relatively high costs of preparing the spray dried clay. The art, therefore, has been continually seeking to effect methods whereby clay filter cakes containing 70% clay can be directly obtained from clay suspensions without the addition of spray dried clay.